How to build a website to upload and download file from blender by python?
Start with downloading

It's a lot easier to get content from the web than it is to post to the net.  So we'll start there.

For more info than you can poke a stick at search google et al with

download files with python


Simple example uses python requests to download this question's page source into blenders text editor.  Any file that can be browsed  can be downloaded in this way.  For this can choose any web hosting.

import bpy
import requests


url = 'https://blender.stackexchange.com/questions/160565/how-to-build-a-website-to-upload-and-download-file-from-blender-by-python'
r = requests.get(url, allow_redirects=True)

t = bpy.data.texts.new("Question")
t.write(r.text)
bpy.context.space_data.text = t


Uploading is trickier, generally requires passwords etc, and is probably outside the scope of BSE.